The present invention relates generally to heat sealing and cutting of thermoplastic films to make plastic bags or the like. A sealing and cutting element, usually in the form of a resistance wire heated by the passage of the electric current, is used to sever and seal the thermoplastic film to form an edge of the bag. The present invention, in particular, relates to a wire support which keeps the wire straight and taut and which minimizes the effects of bending fatigue stresses on the wire.
To make the bag, a double layer of a thermoplastic film is held between clamp members. These members are closely spaced so as to form a narrow canyon. The electrically heated resistance wire moves through the canyon to contact and pass through the web thereby severing and sealing the web to form to a side edge of the bag. During passage through the canyon, the wire must remain straight and taut. If the wire slackens it may contact the side of the canyon which may damage the wire or cause an electrical short.
During the bag making operation the wire is repeatedly heated and cooled. Accordingly it is common in the art to employ springs or other tension control means to maintain the wire straight and taut as it expands and contracts responsive to the repeated cycles of heating and cooling. Where there is a space limitation at the ends of the wire, tension can be maintained by supporting each end of the wire on an upright pivotable arm. The arms are biased to pivot in opposite directions away from the center of the wire to maintain tension on the wire. The drawback of this wire support arrangement is that as the support arms pivot to maintain tension, the wire is subject to a bending moment resulting in stress adjacent the point of attachment to the support arm. Repeated bending at this point, as the arm attempts to maintain tension on the wire, subjects the wire to fatigue failure. This is especially true where the wire is relatively stiff and inflexible, as for example where the wire is rectangular or otherwise elongated in cross section with the narrow side representing the film engaging surface of the wire.
In the present invention there is provided a support arrangement for a heat sealing and severing wire which prevents stress concentration adjacent an attachment point of the wire to a pivotable support arms. This is accomplished by forming the wire so it has a relatively flexible portion and then rigidly supporting the wire so that stresses are referred to this relatively flexible portion. Stresses are distributed over the flexible portion and results in improved resistance to fatigue failures of the wire from these bending stresses.